Underground electrical transmission cables are becoming increasingly popular. Underground cabling may be used for aesthetic reasons but underground cabling may also be used when there are certain space constraints, a high likelihood of severe weather conditions (such as very cold temperatures and/or high winds), issues with electromagnetic interference caused by overhead cabling, or a need to protect the security of the transmission system.
Such cabling may be high voltage electrical transmission cabling used in the transmission of high voltage electricity in an electricity transmission network.
As will be appreciated, the provision of electrical transmission cables underground brings with it many issues which are not encountered with overhead electrical transmission cables.
For example, overhead electrical transmission cables have a relatively large distance of air between the conductors of the cables attached to a pylon or other support structure. The air acts as an electrical insulator (so no separate insulation is necessarily required for many applications) and also as a mechanism of transmitting heat away from the cables.
Underground cables, however, need additional electrical insulation as they may be installed in damp ground material which is a relatively good conductor of electricity. The electrical insulation which is provided around underground cables also inhibits the transmission of heat away from the cables. Furthermore, the cables are not surrounded by air and so heat from the cables is, in any event, transmitted away from the underground cables more slowly.
The heat generated by underground electrical transmission cables is a serious issue which must be addressed in the designing of the underground cabling system. For example, the system design may call for specific cable spacings, and larger diameter cables that would otherwise be needed.
In order to allow for the design parameters to be determined, there is a need to determine the rate at which heat is conducted away from the cables so that the correct design can be determined. In order to provide a controlled rate of thermal conduction, the cables are generally surrounded by a material of known thermal conductivity/resistance properties. The material is generally known as a cable surround material may be formed from a mixture of sand and cement.
The conventional cable surround material provides a predetermined thermal conductivity (and hence a predetermined thermal resistivity or ‘TR’). The higher the thermal resistivity of the cable surround material then the slower heat will be conducted from the cable into the surrounding ground and the larger the diameter of the cable must be—to reduce electrical resistance and hence the heat generated during use.
A lower thermal resistivity would allow cables with respective conductors of smaller cross-section to be used. This means less raw materials are needed for the cables, lower transportation costs and more straightforward cable laying.